In lithographic printing, lithographic ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the hydrophilic surface is moistened with water and lithographic ink is applied, the hydrophilic regions retain the water and repel the lithographic ink and the lithographic ink receptive regions accept the lithographic ink and repel the water. The lithographic ink is then transferred to the surface of suitable materials upon which the image is to be reproduced. In some instances, the lithographic ink can be first transferred to an intermediate blanket that in turn is used to transfer the lithographic ink to the surface of the materials upon which the image is to be reproduced.
Lithographic printing plate precursors useful to prepare lithographic (or offset) printing plates typically comprise one or more imageable layers applied over a hydrophilic surface of a substrate (or intermediate layers). The imageable layer(s) can comprise one or more radiation-sensitive components dispersed within a suitable binder. Following imaging, either the exposed regions or the non-exposed regions of the imageable layer(s) are removed by a suitable developer (processing solution), revealing the underlying hydrophilic surface of the substrate. If the exposed regions are removed, the precursor is considered as positive-working. Conversely, if the non-exposed regions are removed, the precursor is considered as negative-working. In each instance, the regions of the imageable layer(s) that remain are lithographic ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or aqueous solutions (typically a fountain solution), and repel lithographic ink.
“Laser direct imaging” methods (LDI) are used to directly form an offset lithographic printing plate or printing circuit board using digital data from a computer. There have been considerable improvements in this field from use of more efficient lasers, improved imageable compositions and components thereof, and improved processing compositions and procedures.
Various radiation-sensitive compositions are used in negative-working lithographic printing plate precursors as described in numerous publications such as U.S. Pat. No. 7,767,382 (Van Damme et al.) that describes a method of forming lithographic printing plates without rinsing after processing (development). Many other publications provide details about such negative-working radiation-sensitive compositions comprising necessary imaging chemistry dispersed within suitable polymeric binders. After imaging, the negative-working lithographic printing plate precursors are developed (processed) to remove the non-imaged (non-exposed) regions of the imageable layer.
Processing solutions have been designed for each type of lithographic printing plate precursor and imaging chemistry, and thus numerous processing solutions have been uniquely designed for use with each type of imaging chemistry. Some processing solutions comprise one or more anionic or nonionic surfactants, one or more organic solvents, and other addenda to provide desired pH (usually less than 12.5) and processing properties.
For example, EP 2,128,704 A2 (Hirano et al.) describes the preparation of lithographic printing plates from negative-working lithographic printing plate precursors using imagewise exposure and processing with a solution having a pH of 8 to 11.5 and comprising a water-soluble amine, and an anionic or nonionic surfactant or a water-soluble polymer. This same processing solution can be used for gumming so that separate gumming and rinsing steps can be avoided before squeeze rolling to remove excess processing solution, drying, and printing. The exposed and processed precursor is also contacted with the usual gumming solutions. Other details of processing including possible processing equipment are provided in [00242]-[0250].
While there have been great interest and attempts in the art to design processing solutions for the combination of development and gumming (without post-rinsing), it has been found that the exposed and processed precursors can exhibit excessive stickiness, which tends to impede normal pre-press and printing press operations.